What is the pixel error in motion tracking?
The re-projection error is the difference in pixels between where the tracker is placing a point in the image, and where it is being re-projected in the geometry when the scene is Solved.
In the perfect world there should be no error, but in the real world there is usually some degree of discrepancy, given the nature of trackers, optics, and user error.
Usually tracking is considered accurate when the re-projection error is less than 1/3 of a pixel (0.3 pixels).


The number is always an average.
You can see the average per tracker (for however many frames the tracker is active) in the dopesheet

Or you get a report of the average of all bundled tracks for the whole scene at the bottom of the page.

How to see bundles and back projecting points of different colors?
and
Single track with bundle should be selected to define axis
Motion Tracking. What do the graphs represent and how are they useful?